The military angle was just the hook that the highway lobby used to pry money from Congress and the state legislators for a national system of paved highways. In the early days, paved roads were desired by either cyclists or the few rich hobbyists who owned cars. Maybe also some farmers who might find it easier to get their crops to market. None of these roads needed to be a national system for those uses, and the paved roads and other roads suitable for motor vehicles in the US before world war 1 were kind of scattered about the country.
By 1919, there was a nascent highway lobby, and also the Army had a ton of trucks left over from the war with nothing to do with them, so someone got the bright idea of running a transcontinental truck convoy as a joint project between the Army and the highway lobby. The Army could show a "proof on concept" of using trucks to transport stuff long distances, while the highway lobby got some useful publicity. The Army also used the caravan as a recruiting draw, though I'm not sure why the Army needed to be recruiting in 1919, I thought they were shedding soldiers as fast as they could. The convoy took over 2 months to drive from DC to San Francisco. Even back then, you could ride or ship something the same distance by train in 4 days, so advances were clearly needed both in highway construction and vehicle design. I think they spent a lot of time stuck in mud and fixing broken down bridges and trucks (with spare parts rushed to the breakdown sites by trains.)
One of the Army officers participating in this event was a guy named Dwight Eisenhower. This is what started his interest in highways, I guess. At the time the highway lobby was happy enough to get a national system of 2-lane paved highways. A couple of decades later, Eisenhower, now a big shot general in command of all of the Allied forces invading Germany from the west, discovered that the Germans had obligingly built for the invaders a nice system of 4-lane freeways, which some of his commander made good use of. A few years later Eisenhower became President of the United States, and from his military experience with roads, advocated a similar national system of freeways in the United States. (There was already the Pennsylvania Turnpike and New Jersey Turnpike and a few parkways around New York.) While Eisenhower was, of course, inspired by his military experience, I do think that the real impetus for this came from civilian sources. In fact, Eisenhower's experience might cause one to think that freeways might not be the best thing for national defense, as the German ones were very helpful for the American invaders. But, with the longstanding American cultural/political reluctance to spend tax dollars on "internal improvements," marketing this as a "defense" project might have been helpful in bringing a few senators and representatives aboard.
